1. Field of the Invention
This invention relates to a power transmission apparatus of the hydraulic pump type for transmitting torque in accordance with a hydraulic pressure which is produced in response to a difference in rotational speed between two rotary shafts, and more particularly to a power transmission apparatus of the type mentioned which is suitable for use with a four wheel drive apparatus of an automobile.
2. Description of the Prior Art
In a four wheel drive apparatus, for example, wherein front and rear wheels are driven commonly by a single engine, the front and rear wheels sometimes rotate at different rotational speeds because they have somewhat different effective rotational radii or because rolling routes are different not only between the left and right wheels but also between the front and rear wheels when the automobile makes a turn or the like. Such a difference in rotational speed between the front and rear wheels will cause an undesirable tight corner braking phenomenon.
Automobiles of the so-called full time four wheel drive type have been developer, therefore, which include a power transmission apparatus interposed between front and rear wheels for preventing occurrence of a tight corner braking phenomenon and for distributing driving force of an engine to the front and rear wheels in response to a difference in rotational speed between the front and rear wheels.
Such a power transmission apparatus either includes a differential gear (center differential gear) interposed between front and rear wheels for absorbing a difference in rotational speed between the front and rear wheels and a differential locking mechanism for locking operation of the front or rear wheels (or a viscous coupling for limiting operation of the front or rear wheels), or singly includes a viscous coupling.
A power transmission apparatuur of the type which includes a center differential gear is complicated in structure because such a center differential gear cannot readily be reduced in size and will cause an increase in weight of a car body and also in production cost and besides a differential locking mechanism, a viscous coupling or a like mechanism for assuring the four wheel drivability is required.
To the contrary, a power transmission apparatus of the type which singly includes a viscous coupling has such a torque transmission characteristic that the torque transmission gradually decreases as the difference in rotational speed between front and rear wheels increases. Accordingly, when the automobile travels on a road having a very low coefficient of friction such as, for example, a sandy road, a muddy place or a snowy or slushy road, if the difference in rotational speed between the front and rear wheels is not sufficiently great, torque cannot be transmitted to the driven side wheels, and consequently, there is the possibility that a slip may occur at the driving side wheels.
A novel power transmission apparatus has been thus proposed and is disclosed, for example, in British Pat. No. 21 54 522 and U.S. Pat. No. 4,676,336 wherein torque is suitably transmitted from the front wheel side to the rear wheel side by means of a hydraulic pressure mechanism in place of such a center differential gear and a viscous coupling as described above.
The power transmission apparatus includes a hydraulic pump interposed, for example, in a driving system which interconnectes front and rear wheels. The hydraulic pump includes a rotor connected to be rotated in response to one of front and rear wheels, and a casing connected to be rotated in response to the other of the front and rear wheels. The rotor is accommodated in a coaxial relationship in the casing such that the former may rotate relative to the latter at a rotational speed corresponding to a difference in rotational speed which occurs between the front and rear wheels. The hydraulic pump thus accomplished transmission of driving force between the rotor and the casing thereof, that is, between the front and rear wheels of the automobile in accordance with such a characteristic as illustrated by a solid line curve in FIG. 4 by way of a hydraulic pressure which is produced in the hydraulic pump in response to relative rotation between the rotor and the casing. The power transmission apparatus has excellent features, comparing with a power transmission apparatus which employs a viscous coupling, that great driving force can be transmitted with a compact construction and that occurrence of the tight corner braking phenomenon can be restricted significantly.
With the conventional power transmission apparatus described above, however, the difference in rotational speed between front and rear wheel is small when the automobile is running, for example, at a constant speed on a straight road, but within such a region wherein the difference in rotational speed between the front and rear wheels is small, the torque transmission is very low as seen from FIG. 4.
Consequently, as the rotational speed of a driving system for the front wheels varies, gear wheels and like motion transmitting elements of another driving system for the rear wheels often collide with each other to produce rattling noises which sound disagreeably inside and outside of the automobile. Since the greater the torque (total torque of the driving systems for the front and rear wheels), the greater the variation in rotational speed of the front wheel driving system, if the output power of an automobile increases, the automobile will suffer more seriously from such a problem of noises as described above.
Further, when the automobile runs on a road on which sandy places, muddy places and/or snowy or slushy places which all have a low coefficient of friction are locally present, the front wheels will alternately and repetitively brought into a slipping condition and a non-slipping condition, and accordingly a condition wherein the difference in rotational speed between the front and rear wheels is large and another condition wherein the difference is small or almost zero will occur alternately and repetitively. Consequently, a condition wherein a torque is transmitted to the rear wheels and another condition wherein little torque is transmitted to the rear wheels are repeated successively, and as a result, there is the possibility that the rear wheels may slip sidewardly to cause a so-called swerving phenomenon of the automobile. In this manner, the conventional power transmission apparatus has a problem over a driving performance.